The present invention relates to improvement in certain processes for the solvent extraction of oils from oil-containing seeds such as, for example, soybean, cottonseed, corn, peanut, safflower, sunflower, and palm.
The oilseed industry of the United States produces on an annual basis about thirteen million tons of seedoils from roughly one billion bushels of seed crops, predominantly soybean and cottonseed. Essentially all of this oil is recovered from the seeds by solvent extraction. The oils find primary use in foods, e.g., shortening, margarine, cooking oils, and salad oils, while seed meal from which the oil has been extracted, having a high protein content, is generally processed into animal feeds. About two percent of this meal is further refined for human consumption.
In the recovery of seed oils, the industry consumes large quantities of extraction solvent. Hexane has long been recognized as the standard solvent in the industry, due in part to its low cost relative to other solvents and in part to its physical and chemical properties. However, in recent years incentive has developed for the replacement of hexane as the solvent of choice for oilseed extraction. Increasing hexane costs and possibilities of supply shortages account for some of this incentive. In addition, for health and safety reasons, solvent specifications in the industry, precautions to prevent exposure of workers to hexane, and relevant hydrocarbon emission standards may be tightened. Furthermore, interest in producing an upgraded seed meal, for instance a meal suitable for human consumption without the need for secondary extraction or other refining, has given rise to attempts at use of alternate extraction solvents.
Of particular relevance to the present invention is the prior art relating to extraction of oilseeds with an isopropanol-based solvent. Characteristics of isopropanol extraction are generally well known and are described, for instance, by Harris et al. in a three-part publication entitled "Isopropanol as a Solvent for Extraction of Cottonseed Oil" (J. Am. Oil Chem. Soc., November 1947, Vol. 24, p. 370-375; December 1949, Vol. 26, p. 719-723; and July 1950, Vol. 27, p. 273-275).
One recognized problem associated with the use of isopropanol for oilseed extraction relates to separation of the oil/solvent extract resulting from the contact of seeds and solvent. Under conventional practice with hexane, there is produced an oil/solvent extract solution from which solvent is removed by evaporation. The recovered hexane solvent may be directly recycled for further extraction service. However, since the latent heat of vaporization of isopropanol is substantially greater than that of hexane, evaporation of isopropanol from the extract obtained upon extraction of oilseeds with an isopropanol-based solvent is impractical from the standpoint of energy consumption. It is known in the art that an energy efficient solution potentially exists to this problem of recovery of isopropanol extraction solvents for recycle. For instance, in the above-referenced publications, Harris et al describe a method for removal of isopropanol solvent from the extract which does not require its evaporation. It is noted that when aqueous isopropanol of a concentraton of about 91 percent by volume, equivalent to about 88 percent by weight, calculated on the content of isopropanol and water, is used as extraction solvent there results an extract from which solvent can be recovered simply by cooling and phase separating. A solvent phase is thereby obtained which typically comprises approximately 75 percent of the solvent originally introduced into the extraction zone. Of the remainder, a small portion stays with the oil phase while the rest is absorbed by the seed meal during extraction.
Although phase separation of the seedoil/isopropanol-based solvent extract is of substantial advantage with respect to process energy requirements it does not yield a recovered solvent phase suitable for direct recycle to further seed extractions. In addition to desired oils, the oilseeds processed by extraction contain other components, e.g., water, free fatty acids, carbohydrates, phospholipids, sterols, etc., which are extracted by isopropanol solvents and tend to build-up in any phase-separated recycle solvent stream and interfere with extraction performance. It is the general object of this invention to provide an improved process for the treatment of the solvent that is phase-separated from the seedoil/isopropanol extract to obtain a solvent suitable for recycle.
For the conventional process in which oilseeds are contacted with an isopropanol-based solvent to produce an extract that is subsequently cooled and separated into solvent and oil liquid phases, the art provides suggestion that numerous aspects of the solvent composition may be important to process function. Because recycle of phase-separated solvent provides the major part of the extraction solvent in any efficient extraction process, particular attention must be given to these aspects of solvent composition in the processing of recycle solvent according to the invention. It is considered most desirable in the art to have a certain balance of isopropanol and water in the extraction solvent that is contacted with the oilseeds. It is known, for instance, that extraction solvents having a high concentration of isopropanol relative to water, e.g., solvents with isopropanol content greater than about 95% by weight, calculated on total isopropanol and water, may excessively dehydrate the seeds during extraction and cause their disintegration into fines which interfere with downstream processing of both meal and extract. It is also known that one of the recognized advantages of isopropanol-based extraction solvents is their ability to extract and fix certain anti-nutritional toxins (for example, gossypol in cottonseed) that are not removed or fixed by hexane solvents. The efficiency of the removal and fixing of these toxins is known to be related to the presence of significant quantities of water in the solvent. Water in the isopropanol-based extraction solvent is also known to improve meal quality through a denaturing effect on the proteins therein. Furthermore, as a consideration particularly relevant to recycle of the recovered solvent, it is difficult to maintain, under conditions in which water is extracted from the seeds, a concentration of isopropanol in the recycle solvent greater than about 88 percent by weight, based on total isopropanol and water. At this concentration isopropanol forms a constant boiling mixture with water, and solvents with higher concentrations of isopropanol are obtained only by special processing, e.g., azeotropic distillation. On the other hand, although the solvent desirably contains water for these several reasons, it is recognized that the solubility of seedoils in isopropanol-based solvents decreases as the relative proportion of water to isopropanol in the solvent is increased, and that solvents having, for instance, an isopropanol content less than about 75% by weight, calculated on total isopropanol and water, are not effective in accomplishing the desired degree of extraction of oil from the seeds.
The presence in a phase-separated solvent of other non-oil substances extracted from the seeds is also known to influence overall extraction process performance if this solvent is recycled. It is known that build-up in the recycle solvent of impurities such as carbohydrates tends to decrease the solubility of oil in the solvent. However, it has also been observed that high levels of free fatty acids in the solvent enhance oil solubility therein. Still, it has further been found that excessive free fatty acids in the extraction system may increase the miscibility of oil and solvent to the point that it adversely effects the desired extract phase separation. It is also generally the case that relatively high levels of free fatty acids in the solvent result in a phase-separated oil having high fatty acid content. The oil product is then of low quality and subject to chemical instability.
It is the particular object of this invention to provide an energy efficient process which regulates the composition of the solvent that is phase-separated from the extract obtained during extraction of oilseeds with an isopropanol-based solvent to prepare a recycle solvent having a desired balance of the various components, e.g., water, free fatty acids, carbohydrates, etc., extracted from the seeds along with seedoils. In this regard, it is known in the art, e.g., from the work of Harris et al, that the concentration of water in the phase-separated recycle solvent can be regulated by extracting only seeds which have undergone a pre-treatment to adjust their moisture content to a specified level, i.e., ten percent by weight, that is said to be in equilibrium with that of the solvent. Furthermore, build-up of the content of free fatty acids, carbohydrates, and other non-oil substances in the phase-separated recycle stream can be controlled by removal of a bleed stream from the process. No differentiation was attempted between removal of free fatty acids and carbohydrates and other impurities, and success of the process was still said to be dependent to some extent upon free fatty acid content of the particular oilseed feedstock.